This invention relates to a semiconductor intergrated circuit device, and in particular to an earth wiring structure for reducing mutual electrical interference among a plurality of earth wirings included in a semiconductor integrated circuit.
FIG. 1 shows the earth wirings and a wire bonding electrode structure of a conventional semiconductor integrated circuit device, and FIG. 2 is a cross-section of this structure taken along the line II--II in FIG. 1. Numeral 1 denotes one conductivity type single semiconductor substrate (for example, p-type silicon). Numerals 5 and 6 denote island regions among semiconductor layers 20 of the opposite conductivity type (for example, N-type epitaxial layers) which are formed so as to extend downwardly from a main surface of the substrate. These island regions are separated by separation regions 2, 3 and 4 each consisting of, for example, a P-type diffusion layer. Numerals 7 and 8 denote electronic circuits each representing an element (for example, resistors comprised of a P-type diffusion layer formed in the island regions 5 and 6, respectively). Numerals 00, 400 and 500 denote earth wirings connected to the respective electronic circuits (for example, and Al vacuum deposition layer). At 14 is an earth electrode, i.e., a common earth electrode (for example, an Al vacuum deposition layer) for collecting the respective earth wirings at a single point. At 26 is an insulating layer (for example, silicon dioxide). Numerals 9, 10, 11, 12 and 13 denote electrodes. R.sub.0, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9 and R.sub.10 are the intrinsic parasitic impedances of the respective structures. In FIG. 2, numerals 00, 400, 500 and 14 conceptionally denote the same structures as those indicated by the same numerals in FIG. 1. In the drawings, the two electronic circuits 7 and 8 are electrically separated from each other by separation structures including the separation regions 2, 3 and 4. Circuit electrodes 10 and 12 to be grounded are respectively electrically connected to the substrate earth electrodes 9 and 11 formed on the separation regions 2 and 3 via unnumbered substrate earth wirings which run respectively close to the circuit electrodes 10 and 12. Further, the respective circuit earth wirings 00 and 500 are electrically connected to the wire bonding earth electrode 14 for wire bonding. It can be considered that the substrate electrical potentials of the electrodes 9 and 11 are the same as that of the substrate, respectively.
It is well known however that the electric paths of the thus arranged semiconductor integrated circuit have finite impedances R.sub.0, R.sub.5, R.sub.6, R.sub.7 and R.sub.9. In an actual example, the following values have been obtained: R.sub.0 =50m.OMEGA.: R.sub.5 =1.OMEGA.; R.sub.6 =20.OMEGA.; R.sub.7 =20.OMEGA.;and R.sub.9 =50.OMEGA..
Assume now that the respective signal currents I.sub.7 and I.sub.8 flowing from the circuits 7 and 8 are 10 mA and 10 mA and that the potential of the bonding electrode 14 is zero volts. At this time, the equivalent circuit is as shown in FIG. 3, in which impedances considered to be sufficiently small are eliminated. In the equivalent circuit, the potential V.sub.8 at the earth point 12 of the electrode circuit 8 can be obtained as follows: EQU V.sub.8 =R.sub.5 I.sub.5 ( 1) EQU R.sub.5 I.sub.5 =R.sub.0 (I.sub.7 +I.sub.8 -I.sub.5)+(R.sub.6 +R.sub.7 +R.sub.9)(I.sub.8 -I.sub.5) (2) ##EQU1## Let R.sub.0 +R.sub.5 +R.sub.6 +R.sub.7 +R.sub.9 =R.sub.100, then ##EQU2## Substituting the above-mentioned impedances in the actual example and the signal current values into equation (4): EQU V.sub.8 =5.5.times.10.sup.-6 +9.9.times.10.sup.-3 ( 5)
Thus, the earth point of the electron ic circuit 8 is subject to an interference of 5.5 .mu.V due to the influence of the electronic circuit 7. This means that the electronic circuit 8 is subject to an interference of -45.2 dB (5.5.mu.V/1 mV) from the electronic circuit 7, if it is assumed that the electronic circuit 8 equivalently has the performance gm=10 mA/1 mV=10 m.OMEGA.. Generally, it is fully understood that sometimes such an interference level (-45.2 dB) cannot be disregarded.
Thus, the conventional earth wiring structure has disadvantages in that electrical interference is apt to occur between the electronic circuits, resulting in a deterioration in the signal quality due to interference by unnecessary signals, particularly in a high gain high-frequency circuit, somtimes resulting in oscillation which may affect the normal amplifying function of the circuit.